Magnetoresistive devices or sensor elements, which may also be arranged as single elements or also in form of a plurality of interleaved single elements, are employed in numerous applications for contact-free position and/or movement detection today, such as movement direction, rotation direction, rotation speed, velocity of a giver object with reference to the sensor arrangement. Rotation angle sensors are increasingly applied for contact-free detection of rotations of a giver object with reference to a sensor arrangement, particularly in automobile technology, such as for ABS systems, systems for traction control, etc. To this end, rotation angle sensors are often employed on the basis of magnetoresistive elements, such as GMR (giant magnetoresistance) elements, wherein GMR elements are substantially characterized by a permanently magnetizable layer of GMR material.
This permanently magnetizable layer has an electrical characteristic dependent on a magnetic field present, i.e. the resistivity of a GMR layer of a GMR device is influenced by an impacting external magnetic field. In bridge arrangement, rotation angle sensors on the basis of the GMR effect may provide inherent 360° uniqueness of the magnetic field to be sensed and have relatively high sensitivity with reference to the magnetic field to be sensed.
In order to realize 360° detection by means of a magnetoresistive structure out of a plurality of magnetoresistive devices, to detect the rotation direction of a wheel or a shaft with reference to the center arrangement, for example, eight magnetoresistive devices are connected with two Wheatstone bridge arrangements (connected in parallel), wherein one of the bridge circuits has reference magnetizations oriented perpendicularly to those of the other bridge circuit. Within each bridge circuit out of four magnetoresistive devices, the reference magnetizations are arranged in antiparallel manner, so that both bridge circuits provide sinusoidal signals dependent on the rotation angle of an external magnetic field, which are 90° phase-shifted with respect to each other. Via an arctan computation of both output signals, i.e. the output signal of the first and second bridge circuits, the angle over a 360° range can be uniquely determined.
A circuit diagram of a possible connection in form of a double bridge circuit 500 with eight magnetoresistive magnetic field sensor elements is illustrated in FIG. 9. The double bridge arrangement 500 includes a first bridge circuit arrangement 502 and a second bridge circuit arrangement 504, each out of four magnetoresistive single elements 502a-b, 504a-b, the magnetizations of which are indicated with reference to the x-axis and y-axis illustrated in FIG. 9. The first bridge circuit 502 includes two magnetoresistive devices 502a with permanent magnetization antiparallel to the x-axis indicated and two magnetoresistive devices 502b with permanent magnetization parallel to the x-axis. The double bridge circuit arrangement 500 further includes a second bridge circuit 504 comprising two magnetoresistive devices 504a with permanent magnetization in the y direction and two magnetoresistive devices 504b with permanent magnetization antiparallel to the y direction each. The individual magnetoresistive devices 502a, 502b, 504a, 504b are connected, as indicted in FIG. 9, wherein the first and second bridge circuits 502 and 504 are connected to each other in parallel and further connected between a supply voltage and a ground potential.
During the operation of the magnetoresistive sensor arrangement 500 of FIG. 9, the first bridge circuit 502 provides an output signal VX between the two center taps of the first bridge circuit, the second bridge circuit 404 providing an output signal VY between the two center taps of the second magnetoresistive bridge circuit. The connection of the magnetoresistive devices 502a, b and 504a, b described with reference to FIG. 9, allows for the detection of an external, rotating magnetic field over an angle range of 360°. As a function of the rotating, external magnetic field, the sinusoidal output signals VX and VY of the two bridge circuits connected in parallel are obtained, wherein the two output signals VX and VY are each phase-shifted by an angle of 90° with respect to each other.
So as to now characterize the magnetoresistive sensor arrangement 500 illustrated in FIG. 9 as double bridge circuit with a plurality of magnetoresistive single elements with reference to its function parameters, the magnetoresistive sensor arrangement 500 to be examined has to be subjected to a substantially homogeneous test magnetic field. To this end, all materials and arrangements in proximity to the magnetoresistive sensor arrangement, i.e. also the circuit substrate or the sensor wafer of the magnetoresistive sensor arrangement, must have non-magnetic properties so as not to inadvertently influence the homogeneity of the test magnetic field. Moreover, the positioning of the magnetoresistive sensor wafer of the chip package on which or in which the magnetoresistive sensor arrangement to be tested is accommodated goes into the accuracy of the measurement for the characterization of the function parameters of the magnetoresistive sensor arrangement 500.
In the following, it will now be briefly gone into two different procedures for the characterization or examination of magnetoresistive sensor arrangements. In principle, it is distinguished between testing individual circuit chips by means of a laboratory set-up and testing a plurality of magnetoresistive sensor arrangements on a sensor wafer by means of a test apparatus or tester, such as a needle tester.
In a typical laboratory set-up for the examination of a magnetoresistive circuit chip with a magnetoresistive circuit arrangement, the circuit chip is clamped into a non-magnetic fixture, which is in turn mounted into an external test magnetic field as homogeneous as possible. The non-magnetic fixture, the energy supply, and further apparatus in the test magnetic field in the surrounding of the circuit chip to be examined have to be non-magnetic or must not cause influence on the test magnetic field. For the generation of the homogeneous test magnetic field, for example, two stationary mounted magnetic field coils are used, which can generate the test magnetic fields with a magnetic field strength of up to several Tesla. The homogeneous test magnetic field may, however, also be generated by permanent magnets, but these having strong temperature dependency of the test magnetic field provided, which is why permanent magnets are not suited for temperature measurements, i.e. for the detection of the dependence of the sensor behavior over the temperature. Furthermore, it is to be noted that the quantity of the external test magnetic field is assumed as known and must not change over a test series.
In the characterization of magnetoresistive circuit arrangements on a magnetoresistive sensor wafer by means of a tester, the tester used and the entire test set-up have to be adapted to the respective magnetoresistive circuit arrangements to be examined. Thus, it is required to generate an external test magnetic field as homogeneous as possible, which is here also generated by the employment of magnetic coils or of permanent magnets. In a tester construction, the materials within the test magnetic field also have to be non-magnetic or must not cause influence on the test magnetic field. Furthermore, it is to be noted that in such a tester construction the positioning of the wafer with the magnetoresistive sensor arrangement to be examined also has to be exactly correct with respect to the reference angle of the test magnetic field.
After adjusting the external magnetic field and adjusting the orientation of the magnetoresistive sensor wafer with the reference field direction, according to the prior art, the angle measurement by means of the magnetoresistive sensor arrangement attached on the sensor wafer now begins, as it is illustrated in FIG. 9. To this end, the needle card (tester) contacts the sensor wafer, wherein the tester senses the bridge output signals VX, VY of the magnetoresistive sensor arrangement 500 arranged in a double bridge circuit. By means of the sensed sensor bridge output signals VX, VY, the function parameters of the magnetoresistive sensor arrangement to be examined are then determined for the qualification of the magnetoresistive sensor arrangements. The function parameters determined include, for example, the magnetoresistive effect, the amplitude synchronism, an offset signal, orthogonality errors, temperature coefficients, as wells as anisotropy and hysteresis errors, etc.
The known procedures previously explained for the examination of the function parameters of magnetoresistive sensor arrangements are disadvantageous particularly in that the tester or the tester set-up has to be adapted or rebuilt for the examination of the magnetoresistive sensor wafer, wherein it is to be noted that a similarly intensive configuration is required for laboratory measurements. These intensive requirements for examining a magnetoresistive sensor wafer or circuit chip with magnetoresistive sensor arrangements cause relatively high manufacturing costs of the same due to the required calibration steps and the relatively high number of measurement steps in the examination of magnetoresistive circuit arrangements or sensor arrangements.
As already indicated above, at least two magnetic field directions are employed for the characterization of the magnetoresistive circuit arrangements, wherein the test magnetic fields are generated with permanent magnets or coil arrangements. Although the permanent magnets may be mounted relatively easily for the test set-up, it is particularly disadvantageous that the test magnetic field obtained from permanent magnets is temperature-dependent and the permanent magnets have to be rotated in order to obtain the second magnetization direction for the test magnetic field. This rotating of the permanent magnets requires a lot of time in the examination of the magnetoresistive circuit arrangement and is also sensibly employable only as laboratory variant.
Although the current directions may be switched relatively easy with the employment of magnetic coils, it is extremely disadvantageous here that the switching of the magnetic coils requires a relatively long time duration until the inductive test magnetic field has settled.
The German patent application DE 10220911 A1, for example, describes a method for performing a function test of at least one magnetic, particularly magnetoresistive sensor element integrated in a circuit arrangement of equipment during the operation of the circuit arrangement or the equipment, wherein the sensor element is periodically or non-periodically imparted with a magnetic field generated by a magnetic field generation means associated with the sensor element.